FIG. 1 illustrates a network architecture of E-UMTS as a mobile communication system to which the related art and the present invention are applied. The E-UMTS system has evolved from the existent UMTS system and a basic standardization therefor is undergoing in 3GPP. Such E-UMTS system may also be referred to as a Long Term Evolution (LTE) system.
E-UMTS network may be divided into E-UTRAN and Core Network (CN). The E-UTRAN includes a terminal (User Equipment, referred to as ‘UE’ hereinafter), a base station (referred to as ‘eNode B’ hereinafter), a Serving Gateway (S-GW) located at the end of the network to be connected to an external network, and a Mobility Management Entity (MME) for managing the mobility of the UE. One or more cells may exist in one eNode B.
FIG. 2 illustrates a radio interface protocol architecture between UE and base station based on the 3GPP radio access network standard. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer and a network layer, and has vertical planes comprising a user plane for transmitting data information and a control plane for transmitting a control signaling. The protocol layers can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an Open System Interconnection (OSI) standard model widely known in communications systems.
Hereinafter, each layer in the radio protocol control plane in FIG. 2 and a radio protocol user plane in FIG. 3 will be described.
A first layer, as a physical layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to its upper layer, called a Medium Access Control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel. Data is transferred via a physical channel between different physical layers, namely, between the physical layer of a transmitting side and the physical layer of a receiving side.
The MAC layer located at the second layer provides a service to an upper layer, called a Radio Link Control (RLC) layer, via a logical channel. The RLC layer of the second layer supports reliable data transmissions. The function of the RLC layer may be implemented as a functional block in the MAC layer. In this case, the RLC layer may not exist. A Packet Data Convergence Protocol (PDCP) layer of the second layer is used to efficiently transmit IP packets, such as IPv4 or IPv6, on a radio interface with a relatively small bandwidth. For this purpose, the PDCP layer reduces the size of an IP packet header which is relatively great in size and includes unnecessary control information, namely, a function called header compression is performed.
A Radio Resource Control (RRC) layer located at the lowermost portion of the third layer is only defined in the control plane. The RRC layer controls logical channels, transport channels and physical channels in relation to configuration, re-configuration and release of Radio Bearers (RBs). Here, the RB signifies a logical path provided by the first and second layers of the radio protocols for data transmissions between the terminal and the UTRAN. In general, the establishment of the RB refers to stipulating the characteristics of a protocol layer and a channel required for providing a specific service, and setting the respective detailed parameters and operation methods. If an RRC connection is established for the communication of RRC messages between the RRC layer of the terminal and the RRC layer of the UTRAN, the terminal is in the RRC connected mode. Otherwise, the terminal is in an RRC idle mode.
Logical channels are defined between an RLC entity and a MAC entity, and may be divided according to characteristics of data on such logical channels. Transport channels are defined between a physical layer and a MAC entity and may be divided according to how to transmit data on the transport channels.
A random access (RACH) process of a terminal to a base station is described as follows. First, the terminal selects usable random access signature and random access occasion from system information transmitted from the base station via an RRC signal, and then transmits a random access preamble (hereinafter, referred to as message 1) to the base station (Step 1). After successfully receiving the message 1 of the terminal, the base station then transmits a random access response (hereinafter, referred to as message 2) (Step 2). Here, the message 2 includes uplink time synchronization information (Time Advance: TA) with the base station, information (e.g., initial grant) related to uplink radio resource allocation of an identifier C-RNTI to be used in a corresponding cell, and the like. After receiving the message 2, the terminal transmits MAC PDU (hereinafter, referred to as message 3) configured based upon the information related to the radio resource allocation included in the message 2 (Step 3). The base station then either allocates radio resources or transmits an RRC message, according to the message 3 received from the terminal (Step 4).
During data communication between the terminal and the base station, an RACH message 3 is used in the following cases. 1) When the terminal transmits an RRC connection request: since the base station cannot know the existence of the terminal having no RRC connection, the terminal cannot be allocated with radio resources from the base station. The terminal should perform an RACH process for data transmission. 2) When the terminal accesses a target cell: since the terminal has no radio resource allocated thereto during a handover process, the terminal performs an RACH process and transmits a Handover Complete message during the RACH process. 3) When the terminal accesses a new cell under a radio link failure: after having accessed a cell, if a radio environment becomes bad in a state where the terminal failed to receive a handover command and accordingly the connection to the originally accessed cell is disconnected, the terminal re-searches for a cell and accesses a new cell. Here, the terminal performs the RACH in order to transmit data to the new cell. 4) When the terminal in the RRC connected state transmits a radio resource request to the base station: the terminal in the RRC connected state, staying in one cell, performs the RACH for transmitting a radio resource allocation request (Buffer Status Report, BSR) to the base station when the terminal having no uplink radio resource allocated thereto receives data from an upper entity.
Such different cases can be represented in a table as follows.
Table 1
TABLE 1Related LogicalMAC layerMultiplexingScenariochannelneeds UE ID?support required?1. RRC CONNCCCH (TM)NoNoREQUEST2. HO COMPLETEDCCH (AM)YesNo3. HO/RLCCCH (TM)NoNoFAILURE4. BSRN/AYesYes
That is, the RACH message 3 should discriminate each different case as shown in the table. However, the RACH process basically assumes a contention. Namely, in a particular RACH process, the base station should assume that a plurality of terminals can always simultaneously initiate the RACH process. In other words, upon receiving the RACH message 3, the base station should identify which terminal has transmitted the RACH message 3. For CCCH message, the CCCH message itself includes a terminal identifier. Thus, in this case, if the CCCH message is included in a MAC PDU, the MAC entity takes no action but transfers the CCCH message to the RRC.
On the other hand, for BSR, for example, the BSR is transmitted regardless of a message from an upper end. Accordingly, the terminal identifier should be transferred to the base station in a different manner from the CCCH. In particular, a method of effectively identifying the terminal by the MAC entity is required.
For example, when a Dedicated Scheduling Channel (D-SR channel) is allocated to the terminal, whenever data to be transmitted in uplink is generated, the terminal should inform it to the base station via the D-SR channel, and the base station allocates radio resources to the terminal using a terminal specific identifier. In this case, when performing transmission using the allocated radio resource, the terminal does not have to inform its identifier. This is because the base station and a particular terminal have already known how the radio resource is to be used. However, in case of performing transmission via RACH, there is no way to identify a particular terminal, resulting in difficulty of identification.